1. Field of the Invention
The invention relates to a comfort noise generating apparatus for generating a comfort noise corresponding to an output signal of an echo canceller or the like, and adding it to the output signal.
2. Description of Related Art
Generally, an echo canceler is used to cancel an acoustic echo in a hands-free cellular phone used in a vehicle, a television conference, or the like or to cancel a hybrid echo of an international telephone line. The echo canceler has a low level signal shut-off function called an NLP (non-linear process). However, when erasing an acoustic echo or the like by this function, there is a case that a feeling of fracture is generated in association with the voice sound at timings before and after the erasure. To alleviate the feeling of fracture associated to the voice sound, there is a function to add a synthetic noise called a comfort noise. As a comfort noise, it is a general way to add a white noise. That is, a white noise having a flat characteristic in a telephone voice sound band width (300 Hz to 3.4 kHz) is used.
The NLP and comfort noise which are used in the echo canceler are shown in detail in the ITU (International Telecommunication Union) Recommendation (TG.165).
Actually, the noises included in the background of the voice sound, however, do not always have uniform frequency characteristics and their characteristics largely differ in dependence on a use environment. There occurs a case, consequently, where the characteristics of the frequency component of the synthetic noise which is added as a comfort noise are largely different from those of the frequency component of the noises included in the actual voice sound. In this case, there arises a problem that a difference of a tone occurs between the comfort noise which is inserted at the time of the NLP operation and the background noises before and after the comfort noise and this difference causes a feeling of unsuitableness.
To solve the above problems, it is an object of the invention to provide a comfort noise generating apparatus which generates a comfort noise in consideration of characteristics of a noise component in an actual voice sound and which can reduce a feeling of unfitness of the voice sound at the time of an NLP operation.
To accomplish the above object, according to the first aspect of the invention, there is provided a comfort noise generating apparatus for generating a comfort noise according to a voice signal from which an unnecessary component has been removed and for adding the comfort noise to the voice signal, comprising: noise generating means for generating a noise to be multiplexed to the voice signal after the unnecessary component has been removed; signal level analyzing means for measuring a signal power of the voice signal before the unnecessary component is removed; varying means for varying characteristics of the noise which is generated by the noise generating means in accordance with an analysis result by the signal level analyzing means and for forming a comfort noise; and synthesizing means for multiplexing the comfort noise formed by the varying means to the voice signal after the removal of the unnecessary component.
With the above construction, the unnecessary component is removed from the voice signal supplied to an apparatus such as echo canceler, noise canceler, or the like and the resultant voice signal is generated. In this instance, the comfort noise generating apparatus generates and adds the comfort noise to the output signal. In this case, the noise generating means always generates the noise at a predetermined level. The signal level analyzing means measures the signal power of the voice signal before the removal of the unnecessary component. In accordance with a result of the measurement, the varying means varies the characteristics of the noise which is generated by the noise generating means and forms the comfort noise. The comfort noise formed by the varying means is multiplexed to the voice signal after the removal of the unnecessary component by the synthesizing means, so that the voice signal including a natural noise is formed.
According to the second aspect of the invention, there is provided a comfort noise generating apparatus for generating a comfort noise according to a voice signal from which an unnecessary component has been removed and for adding the comfort noise to the voice signal, comprising: noise generating means for generating a noise to be multiplexed to the voice signal after the unnecessary component has been removed; background situation analyzing means for analyzing characteristics of the voice signal before the unnecessary component is removed or a background noise; varying means for varying characteristics of the noise which is generated by the noise generating means in accordance with an analysis result by the background situation analyzing means and for forming a comfort noise; and synthesizing means for multiplexing the comfort noise formed by the varying means to the voice signal after the removal of the unnecessary component.
With the above construction, the apparatus operates as a whole in a manner almost similar to the first invention. According to the invention, the characteristics of the noise are varied by the varying means on the basis of the characteristics of the voice signal or the background noise in place of the signal level of the voice signal. That is, the characteristics of the voice signal before the removal of the unnecessary component or the background noise is analyzed by the background situation analyzing means. In accordance with the analysis result, the characteristics of the noise which is generated by the noise generating means by the varying means are varied, thereby forming the comfort noise.
According to the third aspect of the invention, there is provided a comfort noise generating apparatus for generating a comfort noise according to a voice signal from which an unnecessary component has been removed and for adding the comfort noise to the voice signal, comprising: noise generating means for generating a noise to be multiplexed to the voice signal after the unnecessary component has been removed; signal level analyzing means for measuring a signal power of the voice signal before the unnecessary component is removed; background situation analyzing means for analyzing characteristics of the voice signal before the removal of the unnecessary component or a background noise; varying means for varying characteristics of the noise which is generated by the noise generating means in accordance with analysis results by the background situation analyzing means and the signal level analyzing means and for forming a comfort noise; and synthesizing means for multiplexing the comfort noise formed by the varying means to the voice signal after the removal of the unnecessary component.
With the above construction, the apparatus operates as a whole in a manner almost similar to a combination of the first and second aspects. That is, the signal power of the voice signal is measured by the signal level analyzing means and the characteristics of the voice signal or the background noise is analyzed by the background situation analyzing means. The characteristics of the noise are varied by the varying means on the basis of the analysis results, thereby forming the comfort noise.
According to the fourth aspect of the invention, in the comfort noise generating apparatus of the second or third aspect of the invention, the background situation analyzing means has a signal frequency characteristics analyzing part for analyzing frequency characteristics of the voice signal before the removal of the unnecessary component and for forming control information to control the varying means.
With the above construction, the signal frequency characteristics analyzing means analyzes the frequency characteristics of the voice signal before the removal of the unnecessary component and forms the control information to control the varying means. By controlling the varying means and varying the characteristics of the noise on the basis of the control information, the comfort noise is formed.
According to the fifth aspect of the invention, in the comfort noise generating apparatus of the second or third aspect of the invention, the background situation analyzing means has an LPC analyzing part for LPC analyzing the voice signal before the removal of the unnecessary component and for forming control information to control the varying means.
With the above construction, the LPC analyzing part LPC analyzes the voice signal before the removal of the unnecessary component and forms the control information to control the varying means. By controlling the varying means and varying the characteristics of the noise on the basis of the control information, the comfort noise is formed.
According to the sixth aspect of the invention, in the comfort noise generating apparatus of the second or third aspect of the invention, the background situation analyzing means comprises: a control information storing part for storing control information for allowing the varying means to form a comfort noise corresponding to a background noise in a vehicle which changes in accordance with a change in vehicle situation of an engine rotational speed, a speed, or a flow rate of an air-conditioner which has previously been measured, or engine control information, or the like; and a control information selecting part for selecting an optimum one of a plurality of control information stored in the control information storing part on the basis of the change in the vehicle situation.
With the above construction, on the basis of the change in vehicle situation such as an engine rotational speed or the like, the control information selecting part selects an optimum one of a plurality of control information stored in the control information storing part. By controlling the varying means and varying the characteristics of the noise on the basis of the selected control information, the comfort noise is formed.
According to the seventh aspect of the invention, in the comfort noise generating apparatus of any one of the first to sixth aspects of the invention, the noise generating means comprises: a noise generator for generating a white noise signal at a predetermined level; and a multiplier for multiplying the white noise signal from the noise generator by a gain calculated by the signal level analyzing part, thereby forming the white noise signal according to a level of the voice signal.
With the above construction, the noise generator always generates the white noise signal at the predetermined level. In the signal level analyzing part, the gain is calculated on the basis of the voice signal. In the multiplier, the gain is multiplied to the white noise signal from the noise generator, so that the white noise signal according to the level of the voice signal is formed.